Big Dave means calories.

A calorie (actually a kilo-calorie) is the heat energy necessary to raise one liter or one kilo of water one degree Celsius. A BTU (British Thermal Unit) is the equivalent: the amount of heat energy necessary to raise one pound of water one degree Fahrenheit.

Thin tubes can conduct more heat energy to the water flowing through them, but put up a high resistance to water pressure. Thicker tubes conduct less energy but have lower resistance. Therefore, several thin tubes parallel to each other can lower resistance AND increase conduction.

The more water you can flow through ANY system, the more heat you will add to your pool. The limiting factors are causing leaks (bad) or, possibly cavitation. Don't be fooled by the false adage that you have to run the water slowly through the panels so it can heat up. It SEEMS to make sense but actually doesn't. Again, the confusion is between heat energy and temperature, and we want our solar panels to move heat, ie, calories/BTUs. A very heavy strong flow that's only 1 degree warmer than your pool will STILL heat it faster than a trickle of hot water.

And, just to add to the confusion, as the water in the pool gets warmer, so will the water it feeds to the solar panel, that then STILL comes back warmer, even if only one degree, and warms the pool more!

I've been running solar for 11 seasons now. I've not built my own panels, but the principles are the same.